Starter Problems

For some reason my f150 pickup is going through starters like my kids change their underwear. The original one lasted for a few good years, but any that I replace seem to wear out. I have tried both new and remanufactured ones. I can get about a year out of one then I need to start giving it a tap with a hammer to get it to go. Just wondering if anyone had any input or similar problems with this.

Oh its a 1992 2 wheel drive with a 5.0L engine.

Quick question, when you change out the starter, are the bolts stuck or relatively easy to remove? On ours we found that one bolt loosened up over time, allowing the starter to torque over and drag on the casing. After tapping out the hole we got it tight. Hope it was helpful.